Field of the Invention
The present invention relates to a technique for performing calibration in an image processing apparatus and an image forming apparatus.
Description of the Related Art
In recent years, generally, an electrophotographic image forming apparatus for commercial printing is connected to an image processing apparatus that aims to increase the added value to form a printing system (image forming system). To stabilize the quality of images delivered from this system, the image forming apparatus and the image processing apparatus each have an adjustment function (calibration function). The calibration function is a function for adjusting functions of each apparatus so as to obtain predetermined optimum output of toner or ink components for forming an image on a printout. In general, each apparatus has a lookup table (LUT) for converting an input signal value of each toner or ink component to an optimum output signal value thereof, and by correcting this LUT which is used when executing print data processing, the calibration function is realized. Each apparatus holds information to be targeted so as to form the LUT.
The purposes and conditions of the optimum output vary with the role of each apparatus. The calibration function of the image forming apparatus aims to optimize image forming conditions, such as applied voltage and laser power. On the other hand, the calibration function of the image processing apparatus aims to generate optimum image processing parameters according to conditions including a type of print sheet. Before processing a print job, each apparatus is required to perform correction of the LUT using its calibration function, i.e. calibration, and the calibration functions of the apparatuses are provided independently of each other.
To stabilize the quality of images delivered from the whole system, it is necessary to properly perform calibration of each of the image forming apparatus and the image processing apparatus. Further, in a commercial printing field in which deliverables become commercial products, stable image quality is critically important, and hence in many cases, calibration of theses apparatuses is performed with high frequency.
Normally, for the calibration function of each apparatus, a plurality of target information items are held according to the image formation conditions or the conditions including types of print sheets, and are managed as calibration setting information in association with the image formation conditions, sheet information, and the like. Appropriate calibration setting information is different between print jobs. An operator who uses the printing system performs calibration of each of the apparatuses, separately, according to a type of a print job. To reduce human errors in operations performed by the operator for adjusting the whole printing system, several methods have conventionally been proposed. For example, Japanese Laid-Open Patent Publication (Kokai) No. 2014-113810 has proposed the following method: The image forming apparatus performs calibration, and then analyzes results of the calibration to judge whether or not it is necessary to perform calibration of the image processing apparatus. According to the result of the judgment, the image processing apparatus notifies the operator that calibration of the image processing apparatus should be executed, thereby prompting the operator to perform calibration operation of the image processing apparatus.
Further, generally, on condition that a predetermined number of sheets are printed or the amount of used toner exceeds a predetermined value, the image forming apparatus performs adjustment for making the image quality close to a state obtained by calibration, without using a sheet, i.e. stabilization control including registration correction and correction of the amount of laser light. This makes it possible to keep the quality of printed images close to an ideal state for a long time.
However, the method proposed by Japanese Laid-Open Patent Publication (Kokai) No. 2014-113810 assumes an environment in which the image adjustment is executed in both the apparatuses, independently of each other, according to the configuration of the conventional printing system. Therefore, the workload of an operator who performs calibration of both the apparatuses does not change from before, and the determination of whether or not to actually perform the suggested calibration sometimes depends on determination by the operator. Therefore, since the workload of adjustment of the whole printing system by the operator is not reduced, and it is up to operator's judgment whether or not to execute calibration, there still remains a possibility of occurrence of a human error.
Further, according to the method proposed by Japanese Laid-Open Patent Publication (Kokai) No. 2014-113810, printing which is not associated with the calibration functions is not inhibited during an interval between calibration operations in both the apparatuses. Therefore, printing which is not associated with the calibration functions can be executed during an interval after execution of calibration of the image forming apparatus and before execution of calibration of the image processing apparatus. Here, printing which is not associated with the calibration function refers to printing without using a LUT to be corrected by calibration, for example, a print operation of the image forming apparatus for copy or the like. If such a print operation which is not associated with the calibration function is repeated in the image forming apparatus after execution of calibration, the amount of toner or ink with respect to applied voltage changes e.g. due to aging of toner or ink. If calibration of the image processing apparatus is performed in this state, the calibration is performed based on a state of the image forming apparatus in which the amount of toner or ink with respect to applied voltage has changed. As mentioned above, when a predetermined condition is satisfied, the image forming apparatus executes the stabilization control for making the image quality close to a state obtained by calibration. This changes the state of the image forming apparatus different with respect to the state of the image processing apparatus obtained by calibration, and hence density-gradation characteristics and color output characteristics of the printing system fail to meet expected values. As described above, if printing which is not associated with calibration is performed at an interval between the image adjustment operations in the image forming apparatus and the image processing apparatus, there is a fear that it is impossible to maintain image quality characteristics, such as density-gradation characteristics and color output characteristics of the printing system, in an appropriate state.